add a "raw" function definition interface to cranelift-module (#1400)

* move trap site definitions into cranelift-module

`cranelift-faerie` and `cranelift-object` already have identical
definitions of structures to represent trap sites.  We might as well
merge them ahead of work to define functions via a raw slice of bytes
with associated traps, which will need some kind of common structure for
representing traps anyway.

* cranelift-module: add `define_function_bytes` interface

This interface is useful when the client needs to precisely specify the
ordering of bytes in a particular function.

* add comment about saving files for `perf`

cranelift/module/src/backend.rs

@@ -6,6 +6,7 @@ use crate::FuncId;
 use crate::Linkage;
 use crate::ModuleNamespace;
 use crate::ModuleResult;
+use crate::TrapSite;
 use core::marker;
 use cranelift_codegen::isa::TargetIsa;
 use cranelift_codegen::Context;
@@ -14,6 +15,7 @@ use cranelift_codegen::{binemit, ir};
 use std::borrow::ToOwned;
 use std::boxed::Box;
 use std::string::String;
+use std::vec::Vec;
 
 /// A `Backend` implements the functionality needed to support a `Module`.
 ///
@@ -85,6 +87,18 @@ where
         code_size: u32,
     ) -> ModuleResult<Self::CompiledFunction>;
 
+    /// Define a function, taking the function body from the given `bytes`.
+    ///
+    /// Functions must be declared before being defined.
+    fn define_function_bytes(
+        &mut self,
+        id: FuncId,
+        name: &str,
+        bytes: &[u8],
+        namespace: &ModuleNamespace<Self>,
+        traps: Vec<TrapSite>,
+    ) -> ModuleResult<Self::CompiledFunction>;
+
     /// Define a zero-initialized data object of the given size.
     ///
     /// Data objects must be declared before being defined.